(CBS News) SAN FRANCISCO - Surveillance cameras and cell phones provided the investigators in Boston with thousands of images and the challenge of sifting through them to identify the marathon bombing suspects. So how did they move so fast?
When the FBI posted video of the two suspects, hundreds of thousands of people tried to access the website--part of what's called crowd sourcing, using sheer numbers of people looking at pictures to help solve the crime.
But beyond the numbers, and despite dozens of camera angles and hi-def video, it is the trained human eye that leads to arrests.
"We're gonna track our suspect from camera to camera to camera," said Grant Fredericks, who teaches video forensics at the Digital Multimedia Evidence Processing Lab at the University of Indianapolis. This is where law enforcement agents train to evaluate raw video for agencies, including the FBI.
"This work starts as a labor. It starts as an incredible manual process," he said.
Investigators must scrutinize the video frame by frame. They look for people who display any patterns or abnormalities that signal suspicious behavior.
"Then that person will be tagged based on the clothing and description, direction of travel, GPS information, time of day, gait of walk -- how does the person walk?" explained Fredericks.
The characteristics of the person that catches the investigators eye is then entered into computer programs that try to find the same suspect in videos recorded by other cameras.
When riots erupted in Vancouver in 2011, police collected more than 5,000 hours of video. A team of 50 video forensic experts at the Indianapolis lab spent two weeks searching for suspects. Their work has led to charges against more than 200 people for rioting.
Fredericks expects the examination of the Boston video will continue, even though that the two suspects no longer pose a threat.
"They are going to have to go through all of that video to insure that they are being thorough and that they are not missing anybody," he said.
The value of video forensics goes beyond helping with arrests. In court, the images can provide compelling evidence of the crime.